The present invention relates in general to testing of multi-coil electric components, often referred to herein as multi-coil windings, which have a plurality of series connected wound coils with the individual coils being inaccessible, and, more particularly, to testing individual coils of such multi-coil windings to improve testing sensitivity. While the invention is generally applicable to multi-coil windings, it will be described herein with reference to testing motor stators for insulation defects, winding defects, reversed polarity of poles and the like for which it is being used initially.
A common procedure for determining various defects in multi-coil windings is known as a "surge test." An apparatus for performing a surge test is illustrated in FIG. 1, wherein a high voltage pulse is applied to a multi-coil winding 10, for example a stator, by charging a capacitor 12 via a capacitor charging circuit 22 and then activating a firing circuit 24 to connect the charged capacitor 12 across the multi-coil winding 10 through a high voltage silicon controlled rectifier (SCR) 14. The high voltage 26 on the capacitor 12, shown in FIG. 2, creates an oscillation within the circuit including a resistance 8, the winding 10 and the capacitor 12. The resistance 8 represents resistance of the winding 10, wiring within the circuit and any additional resistance which may be provided to prevent excessive current flow within the circuit. The damped oscillating waveform created by the excited resistor-inductor-capacitor (RLC) circuit is measured across the winding 10 and shown in FIG. 2.
The damped oscillating waveform created by the excited RLC circuit is measured across the series combination of the individual coils, or poles of a stator, of the multi-coil winding by a measurement circuit 20 shown in FIG. 1 since the individual coils or poles are inaccessible. Analysis of the resulting waveform is used to detect any faults that may be present in one or more of the coils. In the case of a stator comprising a plurality of series connector coils, the analysis is best performed by comparison to stators which are known to be good since tolerances in the wire used to form the coils and the coil winding process itself makes comparison to an ideal standard stator effectively impossible.
Unfortunately, even when comparisons are made to a known good stator, if a stator has a large number of coils, contributions to the waveform from one or a small number of defective coils which indicate a stator fault may be "washed out" by contributions to the waveform from the remaining non-defective coils. In addition, relatively wide test acceptance ranges must be set to accommodate the normal process tolerances and resulting variations in waveforms or data from good multi-coil windings so that marginally defective windings are passed without detection. Several factors must be considered when specifying acceptable limits for production testing, such as wire variation, lamination quality, tolerance of process equipment and tolerance of test equipment. The combination of these factors make it difficult for manufacturers to specify surge test limits that will identify defective coils within multi-coil windings having large numbers of coils much less marginally defective coils.
Accordingly, there is a need for improved testing of multi-coil electrical components having a plurality of series connected coils, such as stators. Preferably, such improved testing would provide greater sensitivity to defects within such multi-coil windings without sacrificing yield and provide simplified and more uniform specification of acceptable test limits.